LDR Volume pedal

[Shakey 1047]

Does anyone have a schematic for a LDR type volume pedal (the simpler the better) and what type of ldr to use. I have a couple of old ORP12's. Any help will be much appreciated.
Dave.

[Steve L]

LDR Volume Schematic

Try this one. It is about as simple as they come.
Almost any LDR will work, you will just have to adjust the LED or lamp current to suit your needs.
Obviously if you will be operating it on a battery, lower current is better.

Steve

[Shakey 1047]

Many thanks Steve that will do nicely.
Cheers Dave,

[Mark Hammer]

Actually, it won't do nicely. Take a gander through the schematic and tell me if you see a volume pot or variable control anywhere.

Didn't think so.

When the LED is bright, the LDR's resistance will go low, resulting in minimal signal attenuation by the "virtual pot" formed by the LDR and 100k resistor to ground. When the LED goes dim, and the LDR's resistance goes high, you turn the signal level down.

So, how to vary the LED brightness, and what the heck is a "superbright"? A superbright LED is one that has a higher millicandle (mcd) luminance rating, such that the same current fed to one with a 3000mcd rating results in MUCH more light being emitted than what you would get from one with a 300 or 600mcd rating. I'm not sure what the industry definition is, but in my books, "superbright" is generally anything 2000mcd or higher.

The schematics shows a total of 5.7k or current limiting resistance around the LED. That's actually not very much. Since a 1k change in current-limiting resistance is unlikely to make a huge difference in the brightness of a superbright LED, I'm guessing that the 1k shown is a fixed resistor there just to make sure the LED doesn't fry, and the 4k7 is a variable resistance, making the current-limiting resistance range between 1k and 5k7 total.

Given how little resistance that is, I'm guessing that what they mean here by "superbright" is something more in the range of 1000mcd. The garden-variety red LEDs you see as status indicators on pedals are generally 300-600mcd at best, and with a 2k2 series resistance, they can still get pretty bright.

So, I would suggest getting yourself something in the 1000mcd range, and use a 10k pot with a 50k trimpot in parallel, and a 10k trimpot in series. That way you can tailor the illumination to go from as bright as you need to as dark as you need. Keep in mind that the more current you draw for the LED, the shorter the lifespan of the battery.

[JerryP]

The schematic doesn't show the shutter or curtain which varies the amount of light the LDR can "see" from the LED.
Jerry

[Mark Hammer]

Aaaaaaahhhhhhhhhhh!!

My bad. Morley has used both pots and shutters over the years, and as someone who has repaired one of their shutter-based pedals, I should have known. At the same time, I've seen enough manufacturer-provided factory schematics with errors in them to not take what I see at face value anymore if it doesn't seem to make sense.

But thanks for pointing that out. Much appreciated. My recommendation about using a trimpot to adjust the brightness and current-draw of whatever LED is used still holds, though. At least that's one thing I get to salvage for my efforts.

[Shakey 1047]

Thank you everyone for your input, I think I'll read through this again and make notes, then do some experiments.
Thanks again,
Dave.